A lamp driving circuit for a gas discharge lamp (such as a high intensity discharge (HID) lamp, but not limited thereto) serves to feed the gas discharge lamp with a required amount of current, and receives power itself from a mains voltage source, such as an AC voltage source. Conventionally, such a lamp driving circuit comprises three stages: a rectifier and upconverter for converting the AC input voltage to a higher DC output voltage, a downconverter (forward converter) for converting said DC voltage to a lower voltage but higher current, and finally a commutator switching the DC current for the lamp at a relatively low frequency. In more recent designs, the last two stages (i.e. the downconverter and the commutator) have been integrated into a single stage, referred to as forward commutating stage.
A forward commutating lamp driving circuit may be embodied in a half-bridge commutating forward (HBCF) topology or a full-bridge commutating forward (FBCF) topology. Thus, such a forward commutating stage always has at least one chain of two series-connected power switching elements, such as MOSFET switches, wherein the gas discharge lamp to be driven is coupled to the node between said two switching elements.
In gas discharge lamps, especially in lower power metal halide gas discharge lamps, a speed of commutation of the lamp current must be high. If commutation is slow, the temperature of the electrodes of the lamp could drop too much during commutation because of the low thermal time-constant of the electrodes, and an instantaneous thermionic emission in the cathode phase will be inhibited. This may lead to high lamp voltage peaks after commutation, deterioration of the electrodes, and lamp extinguishment.
US 2005/0062432 A1 discloses a device for operating a high-pressure discharge lamp, comprising control means for controlling at least one power switching element in its switched-on and switched-off states for controlling the power or current supplied to the high-pressure discharge lamp. The control means are adapted to control the power consumed by the lamp by controlling the on-time (Ton) of the switched-on state of the at least one power switching element.
US 2005/0269969 A1 discloses a driver for gas discharge lamps in which a lamp circuit current is sensed to switch the power switching elements of the driver when the lamp circuit current crosses zero. A zero-crossing sensor consists of a small transformer having its primary winding connected in series with the lamp current. The small transformer is already saturated at relatively small primary currents, and comes out of saturation near a current zero-crossing to provide a signal at a secondary winding of the transformer to control the power switching elements.